When I first moved to New England, I lived in a mountainside cabin with no plumbing, no electricity, and no communications for a year. I did, however, still have my pickup truck, then. There was an artesian spring at the base of the mountain, and I own four Blitz (RIP, stupid lawyers) 6.5 gallon jerry cans, plus I picked up a few 5 gallon cans leftover from bulk dish and laundry detergent sales at the local co-op store (which I used only for wash water). Lots of local residents use the spring, so it gets tested every so often for contaminants.
Every ounce, every milliliter of water and wastewater had to be hand carried in and out of the house. My parking area was about 1/4 mile away from the cabin, and I didn't have a wheeled cart to transport the 40 or 50 lb. jerry cans. In winter, when the snow came, I could use a pulk sled, though.
I became very familiar with water conservation, back then. The cabin had a sawdust composting toilet outhouse, which meant no water needed for flushing, but also buckets full of frozen waste and sawdust until a warm spell or the arrival of the next Spring. The cabin was heated with a single woodstove, only, and cooking was either on top of the woodstove or on a 2-burner propane camp stove.
My only way to bathe was heating up some water on one of the stoves, mixing that with room temp water to a comfortable temp (about 1 part boiling to 2 parts room temp is nice), and leaning myself over a galvanized tub to catch the runoff. At one point, I went something like 142 days without a full-body immersion.
Key to reducing water usage for bathing was making sure to clean my bottom with soap and water after every use of the toilet, and cleaning my armpits with soap and water every time I started to detect any odor. I would also keep a 1 qt Zep spray bottle filled with 70% isopropyl alcohol.
I cut up a couple of clearance shelf towels into squares for cleaning rags, which I would then wash in a 5-gallon bucket with a sink plunger, and line dry. I would use those same rags to mop my floor, using a floor scrub brush and a rag under it. I also washed my underwear with the bucket and plunger method.
A couple of times after leaving that cabin, I was homeless, living out of the back of the pickup; fortunately, both times were during the Summer. I bought a 7 gallon Reliance Aquatainer, and made a spout for it with a hose bibb. I would find a seculded area on a sunny day, prop the jerry can on top of the truck, stand on a kitchen dish drainboard to keep dirt off my feet, and shower. I could wash my whole body, including shampooing and conditioning my long hair, with about 2 gallons of water. The artesian spring made that easy, since it is on the side of the road, and I could just pull up with the truck to refill the container at any time of day or night.
I kept my dishes clean by wiping them down with paper towels (which would be composted) and washing them with Simple Green, which rinses very easily to save water. I also kept a 5-gallon bucket with a Reliance Luggable Loo top and a second 5-gallon bucket full of sawdust from one of the local mills for a portable toilet.
When the power goes out, or the water supply becomes unreliable, the two biggest uses of water in the home are always going to be toilets and showers. A couple of 5-gallon buckets with toilet seat tops (like the Luggable Loo) and a supply of raw sawdust are your best friends.
I live in an apartment in the downtown area, now, but I still keep my four Blitz and one Reliance jerry cans filled with fresh water treated with a bloop of bleach. I'm on municipal water, so I don't lose pressure when the power goes out, but the municipal water treatment system can get overtaxed if there is heavy rainfall.
And I still keep the bucket toilet gear, sawdust, and the bucket/plunger washing machine.